Biomass energy is the only “multifunctional” renewable energy which can be converted into gas, solid and liquid fuels and realize complete substitution of fossil energy. Developing with priority alternative petroleum-based liquid fuel and biomass-based high-end products of chemicals (oxygen-containing liquid fuel and high-value chemicals) helps to fundamentally solve the oil shortage of China, protect national energy security and meet the requirements of China's major national development strategies. At present, the mainstream technologies for convening biomass into liquid fuel include a biochemical conversion method and a thermochemical conversion method, wherein, biomass pyrolysis liquefaction technology is one of the biomass utilization technologies with the largest development potential in the thermochemical conversion method.
Compared with biomass raw materials, biomass pyrolysis liquefaction product bio-oil has significant advantages, such as high energy density, easy storage and easy transport. Bio-oil can be directly applied in industrial boilers, gas turbines and other equipment. Refined bio-oil can be used as a substitute for automobile fuel. However, the oxygen content in bio-oil is as high as 30-40 percentage by weight (wt %) (the oxygen content in heavy oil is about 1 wt % in general) and its higher calorific value is 16-19 MJ/kg, less than a half of that of petroleum. Further, due to the existence of a large amount of unstable oxygen-containing compounds, oil will undergo polymerization and decomposition reactions when it is heated to 80° C., so the application scope of oil is greatly limited. Generally speaking, in order to improve its use value, bio-oil must be refined.
Bio-oil quality improvement processes mainly include physical methods and thermochemical methods (catalytic methods). The physical methods include emulsification methods and refining fractionation methods, which may realize effective separation and application of organic components of bio-oil, but do not fundamentally change the shortcomings of bio-oil, including high oxygen content, low calorific value and poor thermal stability. Thermochemical (catalytic) methods mainly include bio-oil catalytic cracking methods and bio-oil catalytic hydrogenation methods. Catalytic cracking may realize deoxygenation of bio-oil and improve the calorific value of bio-oil so as to obtain high-quality liquid fuel and chemicals. The catalytic hydrogenation method of bio-oil may partially deoxidize bio-oil, saturate unsaturated bonds, improve the ratio of polyalcohol and other target products in the liquid product, realize a high carbon conversion rate, improve the overall quality of bio-oil and obtain high-quality biomass-based liquid fuel and high-value platform chemical compounds. American professor Douglas Elliott, an expert in the field of bio-oil quality improvement even pointed out that the bio-oil catalytic hydrogenation process will become an important direction for upgrading bio-oil into “high-quality liquid fuel and chemicals”. It is noteworthy that most of the present methods for upgrading bio-oil quality by thermochemical catalysis (catalytic hydrogenation and catalytic cracking) are directed to all-components of bio-oil (water phase and water insoluble phase or oil phase). Plentiful research has proven that the oil phase (water insoluble phase) of bio-oil is mainly composed of lignin derivatives as well as some oxygen-containing heterocyclic organic matters, and thus has poor thermal stability. During conversion, it is highly likely to cause catalyst coking and loss of catalysis, impairing process stability and continuity. Some methods are for thermochemical catalytic conversion and upgrading exclusively for the water phase or oil phase of bio-oil and do not realize utilization of all-components of bio-oil.
Therefore, conducting “staged conversion” of bio-oil by combining thermochemical catalytic method with other new methods based on the physical and chemical properties of different components (water phase and oil phase) of bio-oil to alleviate catalyst coking and enhance process continuity and stability is the only way to realize high-value, large-scale utilization of bio-oil. The demand is urgent.